It is known to spin polymer mixtures of polyesters and small amounts of a second polymer at high speeds between 1500 and 8000 m/min to form polyester yarns. Defined mixture polymers for this are described for polyester in EP 0 047 464. In addition to a series of acrylic polymers, polymethyl methacrylate is recommended there in particular as a polymeric reagent for polyethylene glycol terephthalate (i.e. polyethylene terephthalate or PET). However, when it is used, thread breaks have been reported at least at a spinning speed of 7500 m/min (cf. ibid example 3).